Highly nonlinear phenomena, where macroscopic changes are triggered by microscopic perturbations, are ubiquitous in complex systems ranging from epidemic spreading to neural networks. In optics, photon avalanching represents a prime manifestation of such highly nonlinear behavior, characterized by a steep nonlinear response (s) following an avalanche threshold. However, this nonlinearity is typically determined by material composition, making this behavior challenging to tune for adaptive sensing, imaging, and neuromorphic applications. Here, we demonstrate giant optical magnetic field gated threshold and nonlinearity in lanthanide doped nanoparticles. We exploit the distinct quantum selection rules of the avalanche process: while the energy looping cycles of excited state absorption and cross relaxation are primarily electrically driven, the seed excitation, ground state absorption, is a mixed transition permitting magnetic dipole interaction. By positioning the emitters within an optical standing wave to spatially partition the electromagnetic field, we tune the local magnetic to electric field balance to selectively regulate the seeding and amplification probabilities. This allows us to demonstrate an unprecedented dynamic range of control over the power threshold and nonlinearity, tuning the order from s = 15 to s = 62. Furthermore, we harness this spatial separation of electric and magnetic fields using tightly focused cylindrical vector beams to locally sculpt the nonlinearity of a single nanocluster, achieving sub wavelength super resolution imaging that substantially surpasses conventional photon avalanche imaging under Gaussian illumination. These findings establish the magnetic component of light as a practical knob to actively program nonlinear optical dynamics, sub diffraction nanoscopy, and critical quantum dynamics at the nanoscale.